Integrated well-log, VSP, and surface seismic analysis of near-surface glacial sediments: Red Lodge, Montana

Jingqiu Huang, Robert R. Stewart, Joe Wong, Carlos A. Montaña

We conducted a series of geophysical surveys to characterize a glacial bench deposit and underlying strata near Red Lodge, Montana. Well logs and VSP data were acquired in a PVC-cased, 115m deep borehole. The multi-offset VSP was undertaken using surface sources (an accelerated weight drop and sledge hammer) with a hydrophone string and downhole, wall-clamping, 3-component geophone. The well logs included measurements of conductivity, radioactivity (gamma ray), temperature, and sonic velocity. Sonic and VSP velocities ranged from 1500m/s in the very near surface to 3000m/s at 85m depth. A distinct black clay layer (with high conductivity, high gamma ray, and low velocity) was penetrated at 85m. High-resolution 2D and 3D seismic surveys, using a sledge hammer source, showed a number of reflectors to about 150ms two-way traveltime. On the L-plot composite displaying well log data, synthetic seismograms, and the VSP corridor stack, a reflection at 80ms correlated with the 85m interface Various other reflections in the VSP and surface seismic data were interpreted to represent glacial deposit layers and water zones (from the perforation logs).